This invention relates to new methods for performing optical coherence-domain reflectometry (OCDR) and optical coherence tomography (OCT). These new methods will potentially increase the stability and robustness and decrease the manufacturing costs of OCDR and OCT systems. The new inventions feature versions of OCDR and OCT which have no moving parts, can be operated at higher speeds, are much more compact than the current state of the art, and will potentially be much less expensive to manufacture in quantity and to maintain in field operation.
OCDR is a method for determining the positions and amplitudes of optical reflections along an optical axis (called an A-scan) by using low-coherence interferometry. OCDR has been used for characterizing fiber and integrated-optics components and biological tissues. In OCT, the optical beam is scanned laterally across the sample surface while recording A-scans, thus obtaining a two-dimensional map (called a B-scan) of the positions and amplitudes of optical reflections as a function of both depth and lateral positions. OCT has been widely applied for cross-sectional imaging both medical and non-medical applications.
Most previous implementations of OCDR and OCT to date have used some variant of the scanning Michelson interferometer design 10 illustrated in FIG. 1 (at right). In this interferometer 10, light from a low-coherence source 11 is split evenly by a fiber coupler 12 into sample and reference arms 13, 14 of the interferometer. Light returning from the reference arm interferes coherently with light returning from reflections internal to the sample 15 only when the relative path lengths from the beamsplitter 12 to the reflections in each arm are matched to within the source coherence length (the xe2x80x9ccoherence gatexe2x80x9d). For typical biomedical and industrial applications of OCDR and OCT, sources with coherence length on the order of 5-15 micrometers are chosen. In normal operation, the reference arm 14 of the interferometer 10 is scanned at a constant or near-constant velocity. Scanning the reference delay in this manner shifts the frequency of the light returning from the reference arm according to Doppler""s equation:                               f          0                =                              V            φ                                λ            0                                              (        1        )            
Here, f is the shift in the reference light frequency, V is the scan speed of the phase delay in the reference arm (Vxcfx86=2s for a mirror 16 translating with velocity s), and xcex0 is the center wavelength of the optical source 11. Light returning from the scanning reference delay is mixed with light reflected from various depths in the sample on the surface of a suitable optical receiver 20. The photocurrent generated in this receiver can be characterized as having three separate components: 1) a direct-current (DC) component resulting from light returning from the reference arm 14 (this is usually the largest component and sets the noise level of the resulting measurement); 2) a DC component resulting from light returning from reflections in the sample 15, and 3) an alternating-current (AC) interferometric component resulting from heterodyne mixing of the light returning from the sample and reference arms 13, 14.
The interferometer 10 has a frequency spectrum centered at frequency f, and contains the desired information about the positions and amplitudes of reflections in the sample 15. Typically, signal processing electronics (or alternatively digital signal processing) are provided to separate the information-carrying interferometric component of the receiver 20 signal from the DC components. The separation of the interferometric component from the DC components is accomplished by band-pass filtering or synchronous detection. The signal processing typically includes detection of the envelope of the interferometric signal (either by envelope detection or by synchronous demodulation), followed by further band-pass filtering to reject extraneous optical and electronic noise.
Thus obtained, the envelope of the interferometric signal as a function of the reference arm position represents a map of sample reflectivity versus depth into the sample, and is typically digitized using an A/D converter or dispayed directly on an oscilloscope or an equivalent device. If a two-dimensional image is to be acquired, the beam is scanned across the sample surface between successive A-scans (or equivalently the sample is scanned under the beam), and the data from all of the A-scans acquired are saved as a two-dimensional data set and displayed as a gray-scale or false-color image.
Although the Michelson interferometer topology of OCDR and OCT is still common, several variations of the basic design which allow for illumination of the sample in transillumination, in reflected or scattered directions other than exact backreflection direction, have been reported and demonstrated in recent years. In addition, several variations of the basic OCDR/OCT design which improve imaging efficiency have been disclosed over the past several years. Alternative interferometer designs which make more efficient use of the source light by using unbalanced fiber couplers to direct more than 50% of source light to the sample have recently been described. These designs make use of non-reciprocal optical elements (e.g., optical circulators) to direct the optimal amount of source light to the sample, and to optimally direct light returning from the sample and reference arms to a detector or detector pair.
A high speed scanning optical delay line (ODL) in the reference arm of an OCT system is needed in order to acquire images rapidly. For a review of ODLs which have been applied in OCT, the reader is referred to Rollins, et al. Previously developed ODLs which have been applied to OCT can be classified into four categories. The first category is the class of ODLs that are based on linear translation of retroreflective elements. The second category varies optical pathlength by rotational methods. The third category consists of optical fiber stretchers. The fourth category is based on group delay generation using Fourier-domain optical pulse shaping technology. A severe limitation on the first, second, and fourth categories of ODLs is that they are implemented in bulk optics, necessitating that the reference arm light must be re-coupled into the reference arm singlemode optical fiber after passing through the ODL. This requirement makes the reference arm the most critical optical component to be aligned in the entire OCT system and systems based on these ODLs are very challenging to design, manufacture, and align. This makes such systems very expensive to manufacture and very time-intensive to maintain. OCT systems based on the third category of ODL exhibit a different set of difficulties associated with static and dynamic birefringence effects in stretching fibers. Thus, there is a need for an ODL which is simple, robust, intrinsically fiber-coupled, and inexpensive to manufacture in quantity. The multi-path reference delay network and multi-path, frequency-encoded reference delay networks described herein both satisfy this need.
Several OCT system designs have been disclosed which allow for increased robustness in system by performing sample illumination and signal collection within several parallel channels simultaneously. Parallel collection can be effected by collection of light from either a) multiple depths in the sample simultaneously (here termed temporally parallel detection, b) multiple scattered directions from the same depth in the sample (here termed spatially parallel detection, c) multiple laterally separated sites on the sample simultaneously (here termed laterally parallel detection, or some combination of a), b), or c). Designs which have been disclosed previously include temporally parallel detection by using a Fourier-domain approach with array detection called chirp-OCT, spatially parallel detection using CCD arrays, and laterally parallel detection using CCD arrays and photorefractive crystals in combination with array detectors. Unfortunately, the primary drawback of all of the parallel detection techniques disclosed to date is that they all depend upon some form of array detection (using, for example, CCDs or photodiode arrays) to collect the multiple channels of optical signals. The drawbacks of array detection in OCT include: 1) the dynamic range of many array detectors (e.g. CCD arrays) is much less than that of photodiode detectors typically used in OCDR and OCT. Since the DC component in the detected signal in OCDR and OCT typically overwhelms the interferometric component by several orders of magnitude, the dynamic range of the array detector may become filled or nearly filled with the DC component of the signal, leaving little SNR (signal to noise ratio) left for the interferometric component. 2) Even for array detectors with high dynamic range such as photodiode arrays, the signal processing which must be accomplished to remove the DC component of the OCDR/OCT signal and to extract the interferometric component with high SNR must also be accomplished in parallel, which requires the creation of very complicated and expensive parallel signal processing channels. Thus, while parallel detection in OCT is desirable from the point of view enabling imaging with increased system robustness by having no moving parts, there is a need for a parallel implementation of OCT which uses a single rather than an array detector and for which the required signal processing can be carried out using simple algorithms. The frequency-encoded parallel OCT and multi-path, frequency-encoded reference delay networks described herein satisfy this requirement.
A method has also previously been disclosed for obtaining en face OCT images (i.e. images which are oriented parallel to the sample surface rather than as cross-sectional cuts through the sample) at multiple depths simultaneously by splitting the reference arm light and providing for two or more different delays in the split reference path, each of the split paths being modulated at a characteristic carrier frequency (Podoleanu). This could be considered as a form of frequency-encoded parallel OCT. The difference between this approach and the present inventions (FIGS. 1 and 2) is that the prior art allows for only a few discrete frequency-encoded optical delays, each of which is detected using a separate frequency-selected channel, for the purpose of providing multiple, axially distributed, en face images. The present invention approach provides for a large number of frequency-correlated depth channels, for the purpose of rapidly acquiring multiple depth scans of the sample (A-scans) by the much simpler signal processing approach of analyzing the frequency spectrum of the light returning from the detector. The resulting frequency spectrum directly represents the depth-resolved reflectance properties of the sample, without any need for further signal or image processing.
These and other objects, aspects, features and advantages will become more apparent as the following description proceeds.
A number of features are described herein with respect to embodiments of the invention; it will be appreciated that features described with respect to a given embodiment also may be employed in connection with other embodiments.
The invention comprises the features described herein, including the description, the annexed drawings, and the claims, which set forth in detail certain illustrative embodiments. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed.
It will be appreciated that the features and principles of the invention may be used in systems other than those disclosed herein for PS-OCT and the like.
Although the invention is shown and described with respect to illustrative embodiments, it is evident that equivalents and modifications will occur to those persons skilled in the art upon the reading and understanding hereof. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims.